We learned back in September at the end of the fourth season of Fate/kaleid liner Prisma Illya 3rei!!that a theatrical film is in production for the property.
The placement is right, however, as the four seasons have all landed in the summer for the last few years.
Property concept: Spin-off of Fate/stay night with Illyasviel von Einzbern as the protagonist, living an alternate life as a school girl.
Sadly, there were no other details to be shared about it nor a title known for it yet.